1. Field of the Invention:
The present invention refers to a process for the treatment of thin films which have properties of electrical conduction and/or of reflection in the infrared, especially of low emissivity, in order to locally modify these properties in a selective manner.
2. Discussion of the Background
Conventional methods for defrosting or demisting of heated glass panes rely on controlled heating, by the Joule effect, of a conducting thin film deposited on a substrate of the pane. The heating is performed by subjected the film to an electrical potential by means of electrical supply leads, generally connected to the film at two of its opposite sides.
Unfortunately, once such a heated glass pane is given a relatively complex geometry, such as a non-rectangular contour and/or a more or less pronounced curvature, it is no longer possible to obtain a uniform heating of the entire area of the pane when the conducting film has substantially constant electrical characteristics at each point. Since these complex geometries occur in the majority of the panes with which automobiles or aircraft are equipped, such as the windscreen or windshield, it is of the utmost importance to guarantee a maximum field of view through the panes for obvious reasons of safety, whatever the climatic conditions.
In order to obtain uniform heating, it is thus necessary to provide zones in the film which have a modified electrical conductivity or, as is frequently done in the field of heated glass panes (or heating panes), a modified resistance per unit area.
However, the solutions applied to date for producing a conductive film having differentiated zones of conductivity or resistance per unit area do not give complete satisfaction.
In French Patent Application FR-A-2 208 005, a process is proposed for the treatment of a conducting film deposited by cathodic sputtering, which enables the thickness of the film to be modified locally during the sputtering operation itself, by varying the number of masks used. The local reduction in thickness in the zones in which the masks are used leads to a controlled increase in the square resistance. (The square resistance R.sub..quadrature. is equal to the quotient of the resistivity .rho. divided by the thickness e:R.sub..quadrature. =.rho./e). The main disadvantage of this technique is that it is often necessary to create zones having square resistances very much higher than the initial resistance of the film. This requires considerable variations of thickness in the film, variations that lead to an overall inhomogeneous and annoying appearance of the pane, especially when viewed in reflection.
French Patent FR-2 584 392 discloses a method consisting of subjecting the conducting oxide film, after Ti has been deposited, to a very intense and rapid heating in an oxidizing or reducing atmosphere.
This treatment has the effect of decreasing (with an oxidizing atmosphere) or increasing (with a reducing atmosphere) the resistivity of the film, without modifying its thickness. This technique, which generally employs burners, may utilize a heating differentiated from one zone to another of the film for the purpose of obtaining different conductivities. It does, however, appear to be better adapted as an after-treatment following film deposition by pyrolysis onto a continuous band of glass, rather than for use following film deposition by a process using vacuum, such as cathodic sputtering.
Thus, a method is needed for preparing a heating pane having differentiated zones of electrical conductivity which preserves the visual appearance of the pane and is applicable regardless of the mode of productivity of the semiconducting film.